This invention relates to a method of making single-mode evanescent-wave couplers having reduced wavelength dependence.
A fiber optic coupler is a device that passively splits or combines light between two or more optical fibers. An evanescent-wave coupler is one in which optical energy is transferred from one optical fiber to another by virtue of the electric field overlap between the two cores of the fibers. Since the evanescent field of an optical fiber is an exponentially decaying field, the cores of the two fibers must be brought into close proximity. In general, evanescent-wave couplers are usually constructed using one of three methods; etch and twist, asymmetric polish and fused biconical taper. In the first two fabrication methods the cladding layers of the fibers are preferentially removed by chemical etching or mechanical polishing techniques. The fibers are then placed in contact with one another and carefully aligned to achieve optical coupling. The fused biconical taper (FBT) couplers are fabricated by heating two optical fibers until they coalesce into a composite waveguiding structure. While the fibers are being heated, they are slowly stretched and tapered. This causes the light in the fiber to spread out far enough into the composite structure where it can be coupled to the other fiber.
With the possible exception of some bulk optic coupler devices, all of the single mode couplers developed to date exhibit coupling ratios that vary substantially with the wavelength of light being used. As a result, the couplers must be fabricated for operation in a specific wavelength region (e.g. 820 nm, 1300 nm and 1550 nm) and are of very limited use in fiber optic systems that operate at more than one wavelength. The coupling ratio of a standard 3dB [50/50] FBT coupler can vary at a rate of 0.1%/nm to 0.3%/nm depending on the type of fiber, the fabrication process and the wavelength. Consequently, a 50% coupler made at 1300 nm will typically exhibit a coupling ratio ranging anywhere from 75% to 90% at 1550 nm. Moreover, a 50% coupling ratio at 1270 nm could increase to as much as 68% at 1330 nm. This behavior can be a serious limitation in many applications since the wavelength tolerance of most laser diode manufacturers is .+-.30 nm.
Recently, D. B. Mortimore described a technique for reducing the typical wavelength dependence of the optical coupling in single-mode FBT couplers (D.B. Mortimore, Wavelength-Flattened Fused Couplers, Electron. Lett. 21, 742 (1985)). Mortimore showed that the wavelength dependence of a single-mode coupler could be reduced by fabricating the coupler with fibers having different propagation constants. Mortimore suggests that the difference in propagation constant can be achieved by using fibers having different diameters or profile or by tapering one of two identical fibers more than the other.
Of the two methods suggested by Mortimore to introduce a mismatch in the propagation constants of the two fibers that comprise the coupler, the seemingly easiest method is to use two different fibers. This approach, however, is not preferred because all fibers are somewhat different and successful results with one particular pair of fibers do not guarantee similar results with another pair. The more attractive approach noted by Mortimore consists of starting with two identical fibers and changing the propagation constant of one of the fibers relative to that of the other.
The wavelength flattened couplers described by Mortimore were fabricated by pretapering one of the fibers. Since the fiber was tapered, it had to be twisted around the second fiber to insure contact prior to fusing the two fibers together. Not only does the twisting procedure tend to result in greater loss and increased sensitivity of the coupling ratio to the polarization of the light, but it also makes it quite difficult to obtain consistent results. The inventors' experience has shown that it is very difficult to control the coupling length and the mismatch in the propagation constants in the coupling region when one fiber is pre-tapered. This is because the change in the propagation constant along the fiber taper is continuously varying in much the same way as the fiber diameter varies.
Therefore it is an object of the present invention to provide a method of making a single-mode evanescent-wave coupler having reduced wavelength dependence using two initially identical optical fibers.
It is another object of the present invention to provide a method of making a single-mode evanescent-wave coupler which is reliable and reproducible.
It is yet another object of the present invention to provide a method in which it is easy to control the coupling length and the mismatch in the propagation constants in the coupling region.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.